There’s a few ways to do this:
1) First, have your resource open to the first page.
2) Next, take a screenshot of the page that you want to display as a thumbnail on your Boutique.
Mac - On an Mac please follow this great link
3) Save the image.
4) In your Dashboard, you can choose the image you’d like to use as a thumbnail directly from your computer. (We suggest offering at least 3 images of the first few pages of your resource so that other teachers will be able to preview the greatness that they are about to receive.)
PC - On a PC running Windows ,7/8/10 please follow this great link